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8th grade Chemical reactions: signs of a new substance, and counting atoms worksheet

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Chemical reactions: signs of a new substance, and counting atoms

Grade 8 Science Worksheet

NameDate

Circle the best answer, or write your answer on the line.

  1. In 4 Fe + 3 O₂ → 2 Fe₂O₃, how many oxygen atoms are in 2 Fe₂O₃, Ari?

  2. In 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂, how many oxygen atoms are in 6 O₂, Bo?

  3. In 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂, how many hydrogen atoms are in 6 H₂O, Esme?

  4. Read this first

    Equation

    2 Cu + O₂ → 2 CuO. An atom-count table sits under it with one row per element and columns for left and right, left blank.

  5. Jun checks an equation for copper joining with oxygen. Is it balanced?

    • No: 3 molecules on the left, 2 on the right
    • Yes: 2 Cu and 2 O on each side
    • No: oxygen disappears
    • No: copper is not on the right

  6. Equation

    Fe + O₂ → Fe₂O₃. An atom-count table sits under it with one row per element and columns for left and right, left blank.

  7. Kira writes the rusting equation without numbers in front. Is it balanced?

    • Yes: 2 molecules can make 1
    • No: Fe and O counts differ
    • Yes: atoms can be added in rusting
    • Yes: each element is on both sides

  8. Equation

    2 H₂O₂ → 2 H₂O + O₂. An atom-count table sits under it with one row per element and columns for left and right, left blank.

  9. A teacher adds yeast to hydrogen peroxide, and it foams. Luz checks the equation. Is it balanced?

    • No: the oxygen gas came from nowhere
    • No: 2 molecules become 3
    • No: water has no oxygen
    • Yes: 4 H and 4 O on each side

  10. Noah adds a drop of food coloring to water, and the color spreads. Is this a reaction?

    • No: the dye only mixed in
    • Yes: new dye atoms formed
    • Yes: the water became a new liquid
    • Yes: a color change is a reaction

  11. A teacher lights a candle while the class watches. Which part is a chemical change, Pia?

    • Wax atoms vanishing as it burns
    • Melted wax hardening again
    • Wax melting near the wick
    • Wax burning into carbon dioxide and water

  12. Ren mixes baking soda and vinegar in an open cup on a scale. The reading drops by about 2 g. Why?

    • Some atoms were destroyed
    • Gas escaped into the air
    • The gas weighs less than nothing
    • The scale got wet

  13. Damp steel wool on a scale weighs a little more after it rusts for a week. Why, Zee?

    • Oxygen has no mass, so it must be dust
    • Iron gets heavier when cold
    • New iron atoms were made
    • Oxygen from the air joined the iron
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8th grade Chemical reactions: signs of a new substance, and counting atoms worksheet – printable
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About this worksheet

Chemical reactions: signs of a new substance, and counting atoms. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

  • I can name signs of a new substance (gas, color, temperature change, a new solid). I know a sign is evidence, not proof: boiling water bubbles too.
  • I can tell a physical change (melting, dissolving, mixing) from a chemical one (rusting, browning, burning).
  • I can count atoms with coefficient × subscript, and check whether an equation is balanced with an atom-count table.
  • I can explain mass in an open and a sealed container: atoms rearrange, none are made or destroyed, and gases have mass.

Standard: MS-PS1-2, MS-PS1-5

Words to know

chemical reaction: Atoms rearrange to make a new substance

physical change: Same substance, new shape or state

reactant / product: What goes in / what comes out of a reaction

coefficient: The big number in front: how many molecules

subscript: The small number after a symbol: atoms in one molecule

balanced: The same count of each kind of atom on both sides

conserved: Kept the same; not made or lost

Common mistakes

Your child added the big number and the small number. The big number counts molecules, the small one counts atoms in each, so multiply.

Your child added every mass, including the one left after the reaction.

Your child thought a gas has no mass. Weigh a sealed bottle of soda, open it to let the fizz out, and weigh again: it gets lighter.

What to say

Ask: what was there before, and what is still there after?

Questions parents ask

What is the most common mistake with chemical reactions: signs of a new substance, and counting atoms?

Your child added the big number and the small number. The big number counts molecules, the small one counts atoms in each, so multiply.

Are these 8th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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